Method for manufacturing semi-hard magnetic material

ABSTRACT

Semi-hard magnetic Fe-Mn system alloys, prepared by powder metallurgical methods, are subjected to cold working and subsequent heat treatment to obtain a product of superior magnetic properties and relatively constant magnetic characteristics, as compared to those products prepared by conventional casting methods. The properties of these magnetic alloys render them suitable for use in hysteresis motors, mechanical channel elements employed in electrical communication systems and the like.

E] States Patent 1191 1111 3,769,100 Watanabe et al. 1 Oct. 30, 1973METHOD FOR MANUFACTURING 3,301,720 1/1967 Griest 148/120 SEMLHARDMAGNETIC MATERIAL 3,444,012 5/1969 Shimizu et al.. 148/120 X 2,152,0063/1939 Welch 75/214 x Inventors: Hisashi Watanabe; Ryuji Watanabe;

Hiroshi Teramoto, all of Tokyo,

Japan Assignee: Altai Electric Company Limited,

Tokyo, Japan Filed: Sept. 10, 1971 Appl. No.: 179,550

Foreign Application Priority Data Sept. 11, 1970 Japan 45179795 Oct. 5,1970 Japan 45/87272 U.S. Cl 148/126, 29/4205, 75/200, 148/12, 148/120,148/121 Int. Cl. C2ll 1/00, H01f l/00 Field of Search 148/120, 121, 126,148/12; 75/200; 29/4205 References Cited UNITED STATES PATENTS 2/1967Adams et a! 75/200 X OTHER PUBLICATIONS Walker, E. V. et al. TheProduction of Grain-Oriented 50:50 Nickel-Iron Magnetic Strip By ColdRolling From Sintered Compacts. ln Powd. Met. No. 4 p. 23-31. (1959) TN695 p54.

Primary Examiner-Carl D. Quarforth Assistant ExaminerR. E. SchaferAttorney-Norman F. Oblon et al.

[5 7] ABSTRACT Semi-hard magnetic Fe-Mn system alloys, prepared bypowder metallurgical methods, are subjected to cold working andsubsequent heat treatment to obtain a product of superior magneticproperties and relatively constant magnetic characteristics, as comparedto those products prepared by conventional casting methods. Theproperties of these magnetic alloys render them suitable for use inhysteresis motors, mechanical channel elements employed in electricalcommunication systems and the like.

6 Claims, No Drawings METHOD FOR MANUFACTURING SEMI-HARD MAGNETICMATERIAL BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates generally to a method for manufacturing semi-hardmagnetic alloys,and more particularly to a method for manufacturingFe-Mn system magnetic materials of the quench hardening type,which aresuitable for hysteresis motors and mechanical channel e1ements,such asare used in electrical communication systems. The term Fe-Mn systemmagnetic materials refers herein to magnetic materials consisting ofless than 20 percent by weight of manganese, less than several percentby weight of additives,such as titanium- ,copper,chromium,vanadium andthe like,with the balance being substantially iron. Such magneticmaterials may further contain less than 20 percent by weight of cobaltfor the purpose of increasing the coercive force and the residualmagnetic flux density of the product.

2. Description of the Prior Art Magnetic materials to be useful inhysteresis motors and the like, should have a coercive force of about 50to 200 Oersteds,and a residual magnetic flux density of about 8,000 to17,000 Gausses. In the past, Alnico alloy systems have been used in anattempt to satisfy this need. The Alnico system, however,has severalinherent drawbacks which have inhibited their full utilization in theart; particularly:

1 the magnet is so hard (more than 600 in Vickers hardness) thatmachinability is poor,thereby resulting in high manufacturing costs,dueto the necessity of abrasive working,

2 they are rather expensive,since they use nickel and cobaltmetals,whose yield is very small,in the preparation of the magnet, and

3 it is difficult to obtain a magnet having a high magnetic flux densityand a low coercive force.

It has been proposed to use Fe-Mn system magnets instead ofAlnico,because of its lower hardness,lower cost and more easilycontrollable coercive force. Particularly, Fe-Mn systems arecharacterized by:

1 a hardness of less than 300 Vickers which renders the magnets easilylathe machinable,thereby resulting in significantly greater machiningprecision,and significantly reduced manufacturing costs,

2 a considerably lower cost for the metals required in producing themagnets,as compared with Alnico systems, and

3 a coercive force which can easily be controlled between the range of20 and 150 Oe,which renders these alloys especially advantageous forapplication as rotor magnets in hystersis motors.

Until now,however,Fe-Mn system magnets have been only prepared bycasting techniques in which Fe and Mn are co-melted with otheradditives,such as Co, in vacuum or in an inert atmosphere, cast into amold and then subjected to hot rolling and cold rolling. The productsare then lathe machined,and heat-treated to obtain the desired magneticcharacteristics.

The Fe-Mn magnet systems produced by such casting methods, however,havenot proven to be entirely satisfactory. For one,it is quite difficult,bycasting techniques,to produce a material of constant constituents,-largely because of the high vaporization pressure of Mn at the castingtemperature (for example,the vapor pressure of Mn may be as high as mmHg at l,500

C.,which can cause as much as 10 percent of the Mn to vaporize).Moreover,the cast magnets are frequently subject to blowholes,caused bythe casting of molten materials into a mold. As a result,the magnets arefrequently characterized by uneven magnetic characteristics.

Another disadvantage of the previously prepared Fe-Mn systemmagnetics,is that they are difficult to mass produce,especially whensmaller magnets are required,since production requires separate castingof each magnet into separate molds. I

Still another disadvantage is that in order to obtain a useableconfiguratiori,the cast material must be lathed extensively,therebyresulting in wasted material. Of course,lathing is still easier thanabrasive machining.

A further disadvantage is the great extent of cold working necessary toobtain the desirable magnetic characteristics.

A need exists,therefore,for an Fe-Mn system magnet which can be producedin a high degree of uniformity and constituent consistency,which isrelatively easy to mass produce,and which does not require extensivemachining.

SUMMARY OF THE INVENTION It is therefore an object of this invention toprovide a novel method for manufacturing Fe-Mn system magnets.

It is another object of this invention to provide a novel method formanufacturing Fe-Mn system magnets which are superior in magneticcharacteristics,and

which demonstrate less variation in magnetic characteristics,as comparedwith those magnets prepared by conventional casting techniques.

It is still another object of this invention to provide a novel methodfor manufacturing Fe-Mn system magnets, which require lowermanufacturing costs and permit mass productivity as compared withconventional techniques.

These and other objects have now been attained by admixing Fe and Mnpowders optionally, with one or more additive powders, such as Copowder, compacting and sintering the powder mixture, as in conventionalpowder metallurgical methods, and thereafter subjecting the product toboth cold working and heat treatment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS According to the presentinvention, a mixture of iron and manganese powders is prepared inamounts of 5-14 wt percent Mn, with the balance being Fe. Other additivepowders, such as cobalt, titanium, copper, vanadium, chromium, silicon,or the like, may be added to the mixture. When 5-14 wt percent Mn isused, tita nium may be used in amounts of 0.6-6 wt percent and copper,chromium, silicon or the like, may be used in amounts of 0 to 0.3 wtpercent with the balance being Fe. When Mn is used, in preferableamounts of 10-13 percent, titanium may be used in amounts of 2-4 wtpercent, Cn, Cr, Si or the like may be used in amounts of 0-3 wtpercent, with the balance being Fe. Moreover, when Mn is used in amountsof less than 30 wt percent, Ti in amounts of less than 5 wt percent withthe balance being Fe, and preferably, when Mn is used amounts of 24 wtpercent with the balance being Fe.

The powder mixture is then compacted under high pressure of from -10ton/cm", and is subjected to sintering temperatures of higher than1,200" C, but lower than the melting point in an inert atmosphere or invacuo. Preferably sintering is effected at temperatures of from l,250 Cto l,330 C.

The compacted and sintered product is then cold worked by conventionalmeans, at a working ratio of greater than 40 percent, with no upperlimit, but preferably at a low cold working ratio of from 40 to 65percent, to provide sufficient magnetic characteristics of the magneticmaterials suitable for hysteresis motors.

If desired, the working processes can be reduced by carrying out sizing"concurrently with cold working. This expedient can lead to reduction inmanufacturing costs.

Since the Fe-Mn system magnets are quench hardening type, which issubject to austenite-martensite transformation, the cold working willcontribute to improvements in magnetic characteristics by creatingstrains in the structure of the magnets. The strains promote thetransformation to the fullest extent and increases the density of thesintered compact.

Following cold working, the material is subjected to heat treatment of450 C to 530 C.

The following table compares the residual magnetic flux density Br(Gauss) and coercive force He (Oersted) of (A) Fe-Mn system magnetssintered and subjected to cold working according to the presentinvention, (B) Fe-Mn system magnets sintered according to the presentinvention but not cold worked, and (C) Fe-Mn system magnets manufacturedaccording to the conventional casting method. The superiority of themagnets according to the present invention will be clearly understoodfrom this table.

Br (Gauss) Hc (Oersted) (A) more than 9500 lO-l50 (B) less than 3000 -50(C) 7000-8000 l0-l 50 If the Fe-Mn magnets of the present invention areheat treated, but not cold worked,or are cold worked,- but not heattreated,the superior magnetic characteristics are not obtainable.

The following advantages are obtainable by the present invention:

1 magnets can be prepared which are highly uniform and which arecharacterized by good component consistency and stable magneticcharacteristics,since only a small amount of the Mn will be vaporizedduring the sintering process as low as 0.1 percent if sintering isconducted at l,300 C. for 5 hours).

2 the process of the present invention provides a high massproductivity,as compared with conventional casting methods,especiallywhere small size magnets,as used in hysteresis motors, are prepared.

3 machining procedures can be eliminated with the resultant reduction inmanufacturing costs and lower loss of materials, since the startingmaterials can be compacted into the desired form in the initial stagesof processing.

Having generally described the invention, a more complete understandingcan be obtained by reference to certain specific Examples which areprovided herein for purposes of illustration only and are not intendedto be limiting unless otherwise specified.

EXAMPLE 1 A mixture of materials,consisting of 87 wt. percent Fe and 13wt. percent Mn, was compacted under pressure and then sintered at l,300C.for 5 hours in an inert atmosphere. The product was then subjected tocold working at a working ratio of 50 percent and heat treated at 500C.for 10 hours. The magnetic characteristics of the magnets thusprepared were found to have a residual magnetic flux density of 9,500Gauss and a coercive force of 60 Oersted. By way of comparison, themagnetic characteristics of the magnet prepared without cold working,butunder the same treatment conditions werezresidual magnetic flux densityof 2,300 and coercive force of 20 Oersted.

The amount of evaporation of Mn in the course of sintering was about 0.1percent of the prepared amount of Mn and is considerably less than the10 percent amount for magnets having the same constituents but preparedby conventional casting methods.

EXAMPLE 2 A powder mixture of materials,consisting of 84.5 wt. percentFe, 12.5 wt. percent Mn,and the balance being additives such as Ti, Cu,V, Cr, Si and the like was compacted and then sintered at l,250 C.for 5hours in an inert atmosphere. The compact was then subjected to coldworking, at a working ratio of 65 percent and heat treated at 500 C.for8 hours. The magnetic characteristics of the magnets thus prepared were:residual magnetic flux density of 9,700 Gauss,and coercive force of 55Oersted.

EXAMPLE 3 A powder mixture of materials,consisting of 77 wt. percent Fe,10 wt. percent Mn,10 wt. percent Co and 3 wt. percent Ti was compactedunder pressure and sintered at 1,300 C.for 10 hours in an inertatmosphere. The compact was cold worked at a working ratio of 65percent,and then subjected to heat treatment at 500 C.for 10 hours. Themagnetic characteristics of the magnets thus manufactured were: residualmagnetic flux density of 14,000 Gauss,and coercive force of 35 Oersted.

EXAMPLE 4 A powder mixture of materials,consisting of wt. percent Fe, 10wt. percent Mn, 10 wt. percent Co, 3 wt. percent Ti and 2 wt. percentadditives such as Cu, V, Cr and Si,was compacted under pressure andsintered at l,300 C.for 10 hours in an inert atmosphere. The compact wascold worked at a working ratio of 65 percent and heat treated at 550C.for 8 hours. The magnetic characteristics of the magnets thus preparedwere: residual magnetic flux density of 12,000 Gauss- ,and coerciveforce of 55 Oersted.

Having now fully described the invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit or scope of theinvention. ACCORDINGLY,

What is claimed as new and intended to be secured by Letters Patent ofthe United States is:

l. A method of preparing a semi-hard Fe-Mn system magnetic materialwhich comprises:

admixing 75 to 87 wt. percent iron powder and 10 to 13 wt. percentmanganese powder,

compacting the powder mixture material,

sintering the compacted mixture material at a temperature of 1200 C. to1330 C. in an inert atmosphere.

subjecting the sintered material to cold working to cause austenite tomartensite transformation, heat treating the material to obtain saidsemi-hard Fe-Mn magnet.

2. The method of claim 1, wherein said powder mixture contains at leastone additive powder selected from the group consisting of: Ti, Cu, V,Cr, Si and Co.

3. The method of claim 2, wherein said additive pow-

2. The method of claim 1, wherein said powder mixture contains at leastone additive powder selected from the group consisting of: Ti, Cu, V,Cr, Si and Co.
 3. The method of claim 2, wherein said additive powder isCo.
 4. The method of claim 1, wherein said cold working is effected at aworking ratio of 40 - 65 percent.
 5. The method of claim 1, wherein saidheat treatment is effected at temperatures of 500* - 550* C.for 8 to 10hours.
 6. A magnet prepared by the method of claim 1 which ischaracterized by a residual magnetic flux density of greater than 9,500Gauss and a coercive force of 10 - 150 Oersted.